1. Field of the Invention
The present invention relates generally to lift pumps used in drilled wells, and specifically to methods and apparatus for pump stabilization and for reducing wear due to sliding contact between lift pump rods and well tubing.
2. Well Drilling and Lift Pumps
Fluids such as water and oil are often recovered by lift pumps from wells drilled in the earth. Lift pumps operate through a reciprocating pump rod within the well tubing; the pump rod may be driven, for example, by a pump jack or windmill positioned over the well. Being relatively inexpensive to buy and operate, lift pumps are widely used for crude oil recovery. For ideal operation, however, the pump rod must remain approximately centered within the well tubing as it moves, thus avoiding sliding contact with the tubing wall which could cause excessive wear of the tubing, the pump rod, the pump or all three. A curved or non-vertical well bore makes undesirable rod-wall contact more likely.
Since no drilled well is precisely straight, rod-wall contact is at least a potential problem in virtually all drilled wells. Where a well head can not be placed directly over the oil deposit, a curved well bore may be drilled intentionally to reach the oil. In such cases, well bore curvature has been increased to as much as 16 degrees per 100 feet of length. On the other hand, even when a straight vertical well is desired, bore curvature can not be totally eliminated. In nominally straight wells, bore curvature actually varies in degree and direction throughout the entire depth of the well.
In wells where bore curvature is intentionally maintained in a single predetermined direction, deviation of the well bore from vertical increases with well depth. A well may be drilled in which the upper portion of the well bore is approximately vertical while the portion below the kick-off point is deviated to a virtually horizontal orientation. In such a well, a lift pump is generally not placed in intentionally deviated portions of the well bore (that is, below the bore kick-off point) because of difficulty in keeping the pump rod from striking the wall. And even in the relatively straight upper portions of such wells, bore curvature is still large enough so that pump rods tend to contact the surrounding tubing wall, especially in relatively deep wells. Additionally, buckling of the rod string on the pump jack down stroke and buckling of the well tubing on the upstroke also tend to cause rod-wall contact.
Regardless of the cause, rod-wall contact results in friction and wear that wastes power during pumping and leads to premature equipment failure and unnecessary cost and delay. An example of equipment damage is that caused to pump seals and bearings due to lateral forces and vibration induced by rod-wall contact. Additionally, frictional wear on rods and tubing may cause pump rods to part and/or it may cause a tubing string to part with the lower section falling to the well bottom.
To avoid these and similar problems, rod guides in the form of split collars or radial standoffs have been applied to the rod string to occupy the space between the rod and the tubing. While the collars effectively prevent rod-wall contact over a limited length of rod, they generally add significantly to the frictional work required to pump the well. They may also substantially restrict the flow of fluid past the collars and increase fluid turbulence, both effects causing significant increases in lift pump back pressures when a rod string is lowered into a well. Excessively high pump back pressures tend to lead to premature failure of pump valves.
Radial standoffs may present less fluid flow resistance than split collars, but they still induce significant fluid turbulence and also generally have less wall contact area than a comparable length split collar. Thus, guide-wall bearing surface pressures tend to be higher with radial standoffs than with split collars, resulting in relatively greater wear. If radial standoffs are lengthened to achieve lower bearing surface pressures, the resistance to fluid flow increases in direct proportion to the length. Both split collars and radial standoffs also tend to slip on the rods due to wall friction forces in combination with their relatively high flow resistance, the latter effect causing significant dynamic pressure differentials across each guide. Split collars and radial standoffs are often made of relatively compliant materials, such as plastic or rubber, which provide shock absorption during movement of the rod string. But these relatively soft materials also tend to wear relatively quickly.
Notwithstanding these problems, wells that are both deep and significantly deviated are often drilled to recover oil sequestered in isolated underground deposits or lying under populated areas. A typical well of this kind was drilled near Giddings, Tex. in 1994. The well bore has a substantially vertical upper portion of about five thousand feet and a total vertical depth of about seven thousand feet, but the total bore length is about sixteen thousand feet. In this well, a lift pump is located near the kick-off point (the five thousand foot level), and more than nine hundred rod guides are required between the pump and the well head. Rod guide replacement in such a well is a major expense. And because of the cumulative friction of so many guides on the well tubing, rod rotators can not be used to encourage even wear. Consequently, holes may be worn in the tubing which then allow fluid circulation below the surface.
Pumping of Viscous and Corrosive Fluids
The increased flow resistance associated with split collars and radial standoffs becomes even more pronounced when viscous fluids are considered. Such fluids are common in crude oil recovery, and they often contain significant quantities of paraffin-like substances which may solidify on the spacers and further restrict fluid flow past them. Paraffin build-up may not be a problem in the lower portions of deep oil wells because temperatures exceeding 150 degrees Fahrenheit keep the paraffin liquified, but in general paraffin is destructive to both down hole and surface equipment (rods, tubing, pump and flow line). In particular, as the oil rises to the surface, it cools. Paraffin in the oil then begins to solidify at a portion of the well called the cloud point, which varies somewhat with the composition of the crude oil being recovered. Pump rod spacers located at or above the cloud point may easily become fouled with solidified paraffin-like substances, substantially increasing fluid flow resistance. To maintain well production, oil heated to about 325 degrees Fahrenheit may be periodically pumped down the well to melt the paraffin, but such "hot oiling" can shorten the service life of certain polymers currently used in rod guides.
A further complication of many oil recovery operations is the presence of significant quantities of hydrogen sulfide gas and water mixed with the crude oil. This combination is highly corrosive and can rapidly degrade the quality of finished metal surfaces and other materials exposed to it. Similarly, electrolytic currents between dissimilar metals can destroy components of the well and/or pumping system.
Thus, a rod guide is needed which will provide an adequate bearing surface in contact with the well tubing without imposing an undue friction load on the pump jack, even in curved bores. Further, spacers should be about as corrosion-resistant as well tubing and designed for low fluid flow resistance (including minimum fluid turbulence). Finally, spacers should be relatively resistant to fouling with solidified paraffin-like substances, adapted for efficient hot oil treatment when required, and characterized by a long service life.